The method according to the invention relates to the filtering of anti-collision alerts, when the aircraft is in proximity to the relief and obstacles, and more particularly relates to aircraft comprising a preventive function for detecting collision with obstacles aimed at preventing aeronautical accidents in which an aircraft that is still manoeuverable crashes on the ground or against an obstacle, doing so, as appropriate, despite prior alerts and alarms.
This type of accident is known in the technical literature by the acronym CFIT derived from the expression “Controlled Flight Into Terrain”. While in the past it constituted a significant proportion of air disasters, accidents of CFIT type are henceforth avoided for the most part by virtue of terrain avoidance manoeuvres performed by crews when prompted by alerts and alarms originating from onboard systems for automatically signalling risks of collision with the terrain and obstacles, known by the term TAWS (the acronym derived from the expression: “Terrain Awareness & Alerting Systems”), which include the GCAS system (the acronym derived from the expression: “Ground Collision Avoidance System”) and the T2CAS system (the acronym derived from the expression “Terrain & Traffic Collision Avoidance System”), that are developed and marketed by the company Thales.
The instruction given to an aircraft crew confronted with a risk of collision with the terrain or obstacles is to engage an avoidance manoeuvre in accordance with a predefined avoidance procedure which corresponds to a pure vertical avoidance manoeuvre termed “Pull-Up”, consisting of a climb using the best performance of the aircraft, a manoeuvre termed “standard avoidance manoeuvre” or else “SVRM” standing for “Standard Vertical Recovery Manoeuvre”.
Onboard equipment signalling, in an automatic manner, flight situations entailing risks of collision with the terrain and obstacles, sufficiently in advance so that an effective vertical avoidance manoeuvre is efficacious has been developed in recent years. Among this equipment, TAWS systems are the most impressive calling as they do upon a so-called FLTA function (the acronym standing for the expression: “Forward Looking Terrain Avoidance”) which looks, ahead of the aircraft, along and below its trajectory vertically and laterally, to see if there is a potential risk of collision with the terrain and obstacles.
The principle of TAWS systems is based on monitoring the penetration of the terrain and obstacles into one or more protection volumes linked with the aircraft on the basis of modelling the terrain overflown. The reliefs of the region overflown are catalogued in a digital map accessible from the aircraft. The position of the aircraft with respect to the region overflown is provided by an item of flight equipment such as: inertial platform, satellite-based positioning receiver, baro-altimeter, radio-altimeter or a combination of several of these sensors. The protection volumes linked with the aircraft are advantageously defined so as to contain a modelling of the standard vertical avoidance manoeuvre trajectory engaged in a longer or shorter timescale on the basis of the trajectory followed by the aircraft as predicted on the basis of the flight parameters delivered by the aircraft's flight equipment, assuming that the aircraft preserves its on-trajectory or ground speed vector. The protection volumes linked with the aircraft are in general two in number, of tiered sizes, the furthest advanced being used to give an alert advising the crew of the aircraft that the trajectory followed will have to be modified in the medium term to avoid the terrain, and the closest being used to give an alarm advising the crew of the aircraft that they must actually engage, as a matter of great urgency, a vertical avoidance manoeuvre.
For further details on the concepts implemented in TAWS systems, reference may usefully be made to American patents U.S. Pat. No. 5,488,563, U.S. Pat. No. 5,414,631, U.S. Pat. No. 5,638,282, U.S. Pat. No. 5,677,842, U.S. Pat. No. 6,088,654, U.S. Pat. No. 6,317,663 and U.S. Pat. No. 6,480,120 and to French patent applications FR 2.813.963, FR 2.842.594, FR 2.848.661, FR 2.860.292, FR 2.864.270, FR 2.864.312, FR 2.867.851 and FR 2.868.835.
However, an operational nuisance potentially generated by such systems is the appearance of an inopportune alert linked with erroneous evaluation of the situation of the aircraft in relation to the terrain and surrounding obstacles. There therefore exists a requirement in operational TAWS systems for an adaptation of the logic for triggering alerts in flight situations for which the conventional methods are unsuitable because of the particular local configuration of the relief and obstacles. This may involve an environment in which the aircraft is made to deploy, procedurally, in constrained flight corridors, of small width and in immediate proximity to the surrounding reliefs.
Through the development of the performance of navigation and guidance systems, such procedures, known for example by the name RNP-0.1 procedure are appearing (RNP is the acronym standing for “Required Navigation Performance” describing the minimum guidance precision required by the complete processing chain of the aircraft in charge of guidance; 0.1 is the width of the prescribed corridor).
In such situations, which will be dubbed “controlled” hereinafter, the aircraft is made to follow a strict trajectory, published by the aeronautical authorities and guaranteed not to conflict with the relief and obstacles. The navigation/guidance systems and their internal checking devices guarantee the current integrity of the flight by monitoring any drifting of the prescribed corridor. In fact, so long as these systems do not detect any conditions necessitating abandonment of the conduct of the procedure, there is no actual operational risk since the procedures have been validated in flight.
Nevertheless, the segregation of the navigation and monitoring systems in aircraft necessitates external monitoring means that are as independent as possible so as to ensure a safety net making it possible to detect possible malfunctions of the navigation and guidance systems and of their internal checking functions.
Taking account of the proximity of the relief and obstacles during the conduct of “controlled” flight phases of guiding and piloting the aircraft, it is possible, according to the context of the aircraft and data intrinsic to the aircraft, such as topographic data, that the anti-collision monitoring system may give rise to a hindrance for the crew. This hindrance is due to too large a number of anti-collision alerts transmitted to the crew which do not always reflect an immediate or actual danger for the aircraft.
The problem therefore consists in reducing the rate of false alerts which cause operational nuisance for the crew. This false alert rate tends naturally to increase as the flight proceeds in proximity to the relief, taking account of:                positional uncertainties;        the granularity of the topographic database;        trajectory assumptions formulated by the monitoring system for estimating the most probable route followed by the aircraft in the forthcoming seconds.        
The realization of this type of mission with the current equipment available on the market is recognized as frequently being subject to inopportune erroneous alert situation detections, thus generating audible nuisance for the crew and appreciable operational consequences. The pilot is induced, in the worse case, to unplug the monitoring device, so reducing the safety level of the mission.
A solution currently proposed by the equipment on the market consists simply in advocating in the flight manual that the audible alerts that arise should be temporarily or definitively removed. This solution in fact reduces the safety of the flight, since the checking of the navigation and guidance means is no longer ensured.